docs: add initial event schema registry for indexers

[Justprotocol]

Overview

As assigned in issue #32, this Pull Request introduces the initial comprehensive documentation registry for contract events (docs/events.md).

Changes Made

• Created a single source of truth for off-chain indexers mapping out the Factory and FarmingPool event structures.
• Documented explicit event topics, expected payload field shapes, and their corresponding Rust types (Symbol, Address, u128, u32).
• Detailed the explicit field behaviors and data payload constraints (e.g., the 9-character symbol limit) to prevent downstream client-breaking indexer drifts.

Closes #32